Verb Number rule for sentences with subordinating clauses

Hi,

According to this website (http://naijawriterscoach.com/concord-of-joint-subjects/), for subject-verb agreement of sentences with subordinating conjunctions :

1. "When you join the subjects in a sentence with any of the subordinated conjunctions, only the subject BEFORE the subordinating conjunction controls the verb."

2. "If and appears first, the rule of and applies, and that means ALL the subjects collectively control the verb."

3. "But if a subordinating conjunction appears first, only the subjects which appear BEFORE the subordinator control the verb."

Are these rules GMAT correct ? What subject-verb agreement rules do we need to know regarding subordinating conjunctions.

Thanks.

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